Toward a Quantum Leap in Sustainable Energy: High-Performance Sodium-Oxygen Battery with Abundant, Low-Cost, and Safe Ingredients.

With the ever-increasing impact of climate change and environmental degradation, there is an urgent need to accelerate the advent of renewable energy with reliable energy storage technology. Sodium (Na) metal is regarded as the ideal anode choice for high-energy-density Na-based batteries, as it possesses a high theoretical specific capacity of 1166 mA h/g and low electrochemical potential of −2.71 V. The combination of Na metal anode and oxygen (O2) cathode leads to Na–O2 batteries with a much higher theoretical energy density (1600 W h/kg) than the state-of-the-art lithium-ion batteries (LIBs) while also utilizing significantly more abundant and, therefore, cheaper resources of Na and O2 (Figure ). However, the commercialization of the Na metal anode is still largely hindered by several challenges, namely, metallic Na dendrite growth, unstable solid electrolyte interphase formation, and large volume expansion. These challenges can be intensified in conventional Na–O2 batteries that use liquid electrolytes as these suffer from undesirable dendrite growth and electrolyte leakage posing a serious safety concern. To boost the practical development of rechargeable Na–O2 batteries, it is necessary to engineer effective strategies to solve the aforementioned problems. A recent study by Jun Chen et al. on Na–O2 chemistry provides an insightful solution toward a quantum leap in high-performance Na-based batteries.[1]

Figure 1

Schematic illustration of the sodium–oxygen battery components: the oxygen cathode and sodium metal anode can be produced through truly abundant resources, while the electrolyte is made of environmentally friendly and safe polymer material. Together, these ingredients contribute to a distinctively sustainable solution for high-performance energy storage.

LIBs have become a ubiquitous part of the modern lifestyle and have been responsible for powering all sorts of portable electronic devices and electric vehicles since their commercialization in the early 1990s. However, there are several major limitations with existing LIB technology. First, LIBs rely on relatively rare and, therefore, expensive resources such as lithium and cobalt, and mining these materials can lead to major ecological consequences. Second, the demand for LIBs is constantly increasing because of the transition to electric vehicles and stationary energy storage; hence, it can result in a shortage of supply that drives up the price of LIBs. Lastly, the state-of-the-art LIBs can deliver about 250 W h/kg, which represents only a small fraction of the energy density as compared to fossil fuels. Therefore, a battery technology which costs less, is more ecofriendly, and has higher performance is needed for a more sustainable energy storage solution. To create such alternatives, it is necessary to look beyond lithium and study new Na-based battery chemistry.[2−5]

In this work, a mechanically robust and high-performance Na–O2 battery with quasi-solid-state polymer electrolyte (QPE), which is composed of a novel blend of a typical polymer material and an organic solvent, was reported. In particular, density functional theory simulations reveal that the fluorocarbon components in QPE are responsible for transferring Na+ with a high ionic conductivity of 1.0 mS/cm. Furthermore, finite element analysis suggests that the unique nanopore pattern and high dielectric constant of QPE can effectively enable a uniform electric field distribution during cycling, thus achieving homogeneous deposition of Na without dendrites (Figure a). Therefore, the fabricated quasi-solid-state Na–O2 batteries exhibit an average reversible discharge/charge efficiency of up to 97% and a negligible voltage decay after 80 cycles at a fixed discharge capacity of 1000 mA h/g. The discharge capacity delivered in this work is about 3 times higher than the state-of-the-art LIB. As a proof of concept, flexible pouch cells were assembled that delivered stable electrochemical performance for 400 h while being under various mechanical stress conditions (Figure b). This work demonstrates the application of the QPE for high-performance flexible Na–O2 batteries, which render a very mechanically robust battery system (Figure c). QPE has the advantages of high conductivity and good mechanical properties as well as the ability to trap liquid electrolyte. The patterned porous structure and high dielectric constant of QPE can induce the uniform distribution of electric field during cycling, which ensures the homogeneous Na+ deposition and inhibits dendrite growth. The excellent hydrophobic behavior and nanopore structure of QPE can effectively protect the Na metal anode from H2O erosion. Meanwhile, the ability to trap organic solvent by QPE can minimize the possible electrolyte leakage issue.

Figure 2

(a) Microscopic images indicate that QPE exhibits a much more uniform pore structure with smaller size to enable uniform Na deposition. (b) Capability for the Na–O2 battery to continue operation while under mechanical stresses at different bending angles. (c) Stable operating voltage of the Na–O2 battery while under various mechanical stress conditions.[1] Reproduced with permission from ref (1). Copyright 2020 American Chemical Society.

In the grand scheme of things, this research fits in the broader picture of Na-based batteries by providing a timely insight for efficiently protecting the Na metal anode from dendrite growth in Na–O2 chemistry. There is still a lack of in-depth theoretical and experimental investigations to fully understand the fundamental mechanisms of uncontrolled metallic dendrite growth, which can induce serious safety issues. This work illustrates that manipulating Na nucleation at specific sites through QPE to direct Na+ deposition for dendrite-free metallic Na anodes is key for a highly efficient Na-based battery system. More importantly, solid-state electrolyte including QPE can be the ultimate solution for a safe Na metal battery by eliminating the flammable organic solvent. QPE has the advantages of scalable manufacturability, mechanical flexibility, and lower interfacial impedance. However, the low bulk conductivity hinders it from being commercially viable unless a small amount of liquid electrolyte is still retained in QPE.

The research on Na+ deposition for Na–O2 chemistry is still in its infancy due to the lack of powerful in situ/operando characterization techniques to understand the initial plating behavior and subsequent dendrite formation process.[6−8] Most studies conducted to date are results-oriented and rely on experimental outcomes to draw scientific insights. However, it is even more critical to draw connections between experiments and theory in an iterative manner to gain a fundamental understanding.

With the promising potential of Na-based batteries, a handful of companies such as Faradion, AGM Batteries, and Natron Energy are already working on commercializing this technology. Although significant progress has been made on interface engineering and solid electrolyte development for enabling Na metal batteries with stable cycling performance, most of the investigations conducted to date operated on relatively low current densities and areal capacities and used very low active material mass loading as per industrial standards. To realize practical energy storage solutions, it is necessary to focus on developing new cell chemistries that can meet real world requirements of applied current density and ramp rate. Additionally, effective solutions for suppressing Na metal dendrites often require costly materials or experimental procedures. Therefore, to bring significant research findings out to the world, it would be worthwhile to develop new strategies that drive down the aforementioned costs.